Thermal sensors that sense the thermal conductivity of a material such as a fluid have been used in a variety of applications including gas chromatography.
In gas chromatography, an unknown gas sample volume is injected into a carrier gas, the unknown gas is separated by the action of a separating column, and the separated sample gas components are transported by the carrier gas to and past a sensor, such as the thermal conductivity sensor, that senses changes in thermal conductivity inherent in the various components of the unknown gas. The thermal conductivity sensor responds to any component of the unknown gas whose thermal conductivity is different than that of the carrier gas. Helium is frequently used as the carrier gas because of its exceptionally high thermal conductivity.
The output of the thermal conductivity sensor peaks as each gas component passes by the thermal conductivity sensor and these peaks serve to identify each of the components of the gas by their elution timing and their concentration by their areas under the corresponding peaks. Thermal conductivity sensors available today for performing the above functions are bulky and expensive.
Present absolute thermal conductivity sensors that sense the thermal conductivity of fluids, such as gases, respond to changes in the chemical compositions of the fluids, which is generally the sensing objective. However, these present absolute thermal conductivity sensors also respond to changes in temperature, pressure, orientation, acceleration, vibration, rotation, and flow, which is generally an undesirable response of these absolute thermal conductivity sensors.
Flow disturbances, which result in an erroneously larger thermal conductivity being sensed, can be minimized by judicious sizing of the sensor housing in exchange for some loss in the speed of response. However reducing the undesirable influence of temperature, pressure, humidity, orientation, or rotation typically requires the use of additional temperature, pressure, and/or orientation sensors.
Moreover, differential thermal conductivity sensor assemblies have been used in Gas Chromatography systems where one thermal conductivity sensor is in contact with the outlet carrier gas stream carrying the separated components of an injected sample and the other thermal conductivity sensor is in contact with the inlet pure carrier gas stream. It is thought that, by placing the two thermal conductivity sensors in close proximity to one another, temperature differences experienced by the sensors are minimized and the sensors are exposed to the same flow rate. The sensors are typically oriented in the same direction.
However, the two thermal conductivity sensors in this system may be exposed to different, but steady pressures due to the pressure drop through the separation column of the gas chromatography system. Thus, such gas chromatography systems produce outputs that contain an undesired error component due to this pressure drop. Moreover, available differential thermal conductivity sensors used in these systems are typically manufactured in low volumes, with specially designed hot wire anemometers which result in the systems being very costly.
The present invention solves one or more of these or other problems.